In such a trap, especially one installed in a basement or some other part of a dwelling, it is necessary to block the escape of sewer gases by allowing a pool of water to accumulate on the bottom of the trap beneath an outlet which the runoff can reach only by way of that pool. Certain regulations have been established regarding the effective height of the pool, i.e. the distance between its level and a submerged inlet of a conduit leading upwardly to the trap exit. Thus, as a rule of thumb, that effective height should be not less than the diameter of the exit port. In practice, this height may be 100 mm or more. It is also important to make the flow path through the conduit so wide as to prevent any congestion from developing within the trap when the water enters at a high rate almost equaling the capacity of the drain pipe which carries it off to the sewer. This means that the cross-sectional area of the path should at no place be less than that of the outlet.
In order to facilitate the cleaning of a trap that has become filed with sludge, it has already been proposed (see, for example, German utility model No. 1,958,096) to provide a generally elbow-shaped conduit whose outlet end can be removably fitted into the exit port of the trap and which is internally provided with an upstanding weir allowing the water to accumulate within that conduit to a level higher than the bottom edge of the outlet opening. The flow path left free above the weir, however, is relatively narrow and is susceptible to clogging by muddy water. The insertion of such a fitting into an exit port is not always convenient; it can also happen that the fitting sticks to the port and can be extracted only with difficulty. Problems arise, furthermore, in attempting to insure a fluidtight seal between the fitting and the tubing that forms the port, especially after repeated removals and reinsertions. Without a close fit, however, the escape of sewer gases into the trap and thence into the atmosphere cannot be effectively prevented.